Transferring cargo off or onto a vessel at sea can be challenging, not least in difficult weather and wave conditions. In a typical scenario, cargo may be stored on an offshore storage facility which may be long-term moored in a selected location offshore. From time to time, the storage facility may receive visits from a ship. The cargo may then be transferred from the storage facility and loaded onto the ship. The ship may then depart and transport the cargo to another destination, e.g. a receiving facility onshore or the like. The offshore storage facility may itself be a floating vessel in the form of a ship. It may for instance be spread moored in a fixed orientation, or may be turret moored so that it may rotate and “weathervane” about a mooring point into an equilibrium orientation in response to forces imparted by waves and/or wind. The storage facility may be later replenished with new cargo to replace that which has been loaded onto the visiting ship. In order to transfer the cargo between the storage facility and the ship, it is normally sought to bring the ship alongside the storage facility into a suitable location so that cargo can pass between them using loading/offloading equipment. To this end, various mooring and loading systems are known.
In particular, side-by-Side (SBS) loading systems are well known in the oil and gas industry. Such systems are typically used for loading cargo in the form of fluid or gas, such as liquefied petroleum gas (LPG) or liquefied natural gas (LNG) from a turret moored or spread moored floating production storage and offloading (FPSO) or floating liquefied natural gas (FLNG) facility (ship-shaped floating installation). The SBS system can be characterized by a visiting ship, e.g. a crude oil tanker, LPG carrier or LNG carrier that is moored alongside the floating installation by use of soft fenders and mooring lines between the units. In order to bring the ship alongside the floating installation, tugs are typically used. It is for example common for 3 or 4 tugs to be used for the mooring operation. Once moored, a fluid transfer system between the two vessels may then be connected. The fluid transfer system may typically use loading arms or hoses which may allow some movement between the vessels during the transfer operation. Typically, the transfer operation may take place over a period of around 24 hours.
The mooring operation using tugs can have limitations and challenges, and may suffer particularly in difficult wave, wind, and current conditions. In order to position and moor a visiting ship, e.g. an LNG carrier (LNGC), tugs may push against a side of the ship to urge the ship toward the mooring position alongside the storage facility, while the ship may typically have a heading toward the oncoming waves. In order to push the ship sideways, the tugs (operating in “push mode”) may generally be oriented beam on to the waves and this can result in significant roll motions of the push tugs. Large forces may be generated between the tug and the side of the ship due to the tug motions in waves. In addition, the bollard pull efficiency of the tugs when operating in high waves is reduced due to propeller “ventilation”. Thruster wake from tugs may also hit the side of the ship leading to additionally reduced thruster efficiency. As a result, the typical maximum wave height during berthing of an LNGC vessel can be Hs 2.0-2.5 m (significant wave height) or significantly less depending on the wave periods (long period swell can in particular reduce the operational limit).
There are now means of reducing the roll motion of LNG carriers. However, if the ship is rolling, high mooring line loads can be imparted, and slosh damage may occur in the tank (membrane) containment system of the LNG carrier when partially filled.
It can be appreciated that some hazardous kinds of fluid cargo such as LNG or LPG can present a fire or explosion risk. In the event of an emergency during transfer, it may be necessary to remove the ship quickly from the storage facility. In an emergency escape situation of this kind, e.g. if an explosion or fire occurs, typically two tugs will be used to pull the LNGC safely away from the FLNG, requiring mooring lines from the tugs to be connected, and release of the ship from the storage facility. These operations can be time consuming and inconvenient, and they may form operation critical elements of the system. Thus, there may be requirements for several tugs to be located nearby and potentially be engaged if required during the transfer process, which can be inconvenient. In difficult weather, it can be a challenge to even utilize tugs at all in the mooring and disconnect/emergency activities mentioned above.